Polymers having a terminal functional group are known to give cured products excellent in heat resistance and durability, among others, upon crosslinking either by themselves or in combination with an appropriate curing agent. Typical examples among them are alkenyl-, hydroxy- or crosslinkable silyl-terminated polymers. Alkenyl-terminated polymers are crosslinked and cure upon use, as a curing agent, of a hydrosilyl-containing compound or upon application of a photochemical reaction. Hydroxy-terminated polymers, when reacted with a polyisocyanate, form a urethane bond and cure. Crosslinkable silyl-terminated polymers, when they absorb moisture in the presence of an appropriate condensation catalyst, give cured products.
As examples of the main chain skeleton of such alkenyl-, hydroxy- or crosslinkable silyl-terminated polymers, there may be mentioned, among others, polyether polymers such as polyethylene oxide, polypropylene oxide and polytetramethylene oxide; hydrocarbon polymers such as polybutadiene, polyisoprene, polychloroprene, polyisobutylene, and hydrogenation products derived from these; and polyester polymers such as polyethylene terephthalate, polybutylene terephthalate and polycaprolactone. Such polymers are used in various applications depending on the main chain skeleton and the mode of crosslinking.
Apart from those polymers illustrated above which are obtainable by ionic polymerization or polycondensation, those vinyl polymers having a terminal functional group which are obtainable by radical polymerization have scarcely been put to practical use. Among vinyl polymers, (meth)acrylic polymers have high weathering resistance and transparency, among others, which cannot be expected of the above-mentioned polyether polymers, hydrocarbon polymers or polyester polymers, and those having alkenyl or crosslinkable silyl groups on side chains are used in high weathering resistance paint compositions and the like. On the other hand, the polymerization control of vinyl polymers is not easy owing to side reactions and it is very difficult to introduce a functional group terminally thereinto.
If vinyl polymers having a functional group at a molecular chain terminus can be obtained in a simple and easy manner, there will be obtained cured products excellent in physical properties as compared with those having an alkenyl group on side chains. For that reason, investigations have been so far made by a large number of researchers to develop a process for producing them. However, it is still not easy to produce them on a commercial scale.
In Japanese Kokai Hei-05-255415 is disclosed a process of synthesizing (meth)acrylic polymers having an alkenyl group at both termini by using an alkenyl-containing disulfide as a chain transfer agent and, in Japanese Kokai Hei-05-262808, there is disclosed a process of synthesizing (meth)acrylic polymers having an alkenyl group at both termini which comprises synthesizing a (meth)acrylic polymer having a hydroxy group at both termini using a hydroxy-containing disulfide and then introducing an alkenyl group at both termini by utilizing the reactivity of the hydroxy group. However, it is not easy to introduce an alkenyl group into both termini with certainty by these processes. For terminally introducing a functional group with certainty, the chain transfer agent must be used in a large amount, which raises a problem from the production process viewpoint.
Separately, the present inventors have already invented a process of introducing an olefin group terminally into a vinyl polymer by adding, after polymerization of a vinyl polymer, a compound having a polymerizable alkenyl group and an alkenyl group low in polymerizability to thereby reacting the polymerizable alkenyl group with the polymer terminus. By this process, however, it is not easy to introduce only one olefin into a terminus with certainty even when the polymerization proceeds in a living mode. In particular, addition of the olefin at a stage at which the polymerizable monomer still remains results in random copolymerization, which makes it more difficult to control the structure.
Accordingly, the present invention has an object to have a vinyl polymer having a terminal functional group, a process for producing the same and a curable composition comprising said polymer.
It is known that, generally, unactivated olefins, such as α-olefins, is not polymerized in the manner of radical polymerization. The same also applies to living radical polymerization, which has recently been investigated actively.
As a result of intensive investigations, the present inventors found that when an inactivated (low polymerizability) olefin is added to a living radical polymerization system, approximately one molecule alone adds to the growing terminus and, by utilizing this finding, invented a process for producing polymers having various terminal functional groups.